1. I think that Merry doesn't exactly want to stop Frodo, but if Frodo leaves him and Pip, then the situation will be out of their hands. After it is out of their control, they realize how dangerous it really is. Merry even says, "It seemed different so far away, in the Shire, or in Rivendell." Merry and Pip, in a way, had control in the Shire, or even in Rivendell, when they were able to choose their paths; now, that choice is being taken away from them, and it is only then that they realize how dangerous it is. They know that they might not have had much effect on the destruction of the Ring, or Frodo's safe return; they could even have been a hindrance; but there is a sort of comfort, I think, in knowing that you have control over someone (or something), or you are keeping watch over them. Now that Merry and Pip have lost their portion of protectiveness over Frodo, they are powerless to do anything for him, and it is in this act of stepping back that, for the first time, they realize how vital and dangerous the mission really is. (I think I just said the same thing about fifty times...)

2. Maybe they were all just glad to get some peace and quiet...plus, I suppose the Ring and its bearer will tend to get more attention than the people trying to get it, since the latter are much more common....(I really have no idea for this question....sry!)

3. Aragorn says, "I wonder?" before deciding against making any decisions for Frodo, which maybe suggests that he simply changed his mind- plus, he's just listened to Sam's advice ("you speak more wisely than any of us, Sam"), so I think now he has a slightly better understanding of Frodo from a friend's perspective, as opposed to the hobbit that happens to be carrying the Ring (I'm exaggerating slightly, but still)- so he understands a little more that Frodo should be able to make some decisions, too, now that he's heard from the perspective of a friend (haha, either that or they're all hypocrites)